Bank note processing machines are used for checking bank notes as to their properties, such as authenticity, state, kind of bank note etc. Dependent on the result of the check, the bank notes can be, for example, accepted, sorted, stored, destroyed etc. Here the recognition of forged bank notes is of particular importance. For recognizing bank notes and for differentiating authentic bank notes from forgeries, usually from authentic bank notes criteria or comparative data are derived that permit a recognition of the individual bank note as to currency and denomination and its authenticity. If for specific bank notes, i. e. for a specific denomination of a specific currency, forgeries have become known, these forgeries too are used for deriving comparative data. For this purpose authentic bank notes and forgeries are processed with a bank note processing machine, in order to produce data of the sensors of the bank note processing machine for authentic bank notes and forgeries. Then from the data of the sensors, in sometimes very elaborate processing steps, comparative data for recognizing the bank notes and checking their authenticity are derived.
But it has turned out that in particular with the assessment of the authenticity of bank notes there can arise special problems, if in addition to the forgeries taken into account when producing the comparative data, new types of forgeries emerge. In these cases it can occur, that these new types of forgeries are not recognized and thus are judged as authentic bank notes.
It is obvious that in the described cases it is possible to derive comparative data once more, taking into account also the new types of forgeries beside the authentic bank notes and hitherto known forgeries. Such a procedure, however, has the disadvantage that the entire comparative data have to be derived at least for the kind of bank notes concerned, i. e. the denomination of a specific currency concerned. Moreover, at least the comparative data for the specific kind of bank notes have to be replaced in the bank note processing machines. Here incompatibility problems with the comparative data of the other kinds of bank notes may arise. In order to reliably avoid these problems, usually the entire comparative data are freshly produced, in order to replace the comparative data hitherto used in the bank note processing machines. This, however, requires a considerable effort, since great data volumes have to be mastered. Due to the described considerable effort with the known proceeding the further problem arises that new types of forgeries cannot be taken into account as soon as shortly after their occurrence, since deriving and producing the comparative data already demands a considerable time.